#18 "Leaf Tissues"

Color Code For This Page

• Colorless: epidermal cells (a), air space (h), bulliform cells (l), subsidiary cells (n), hypodermis (o), and transfusion tissue (q)
• Dark green: guard cells (b), monocot and conifer mesophyll (c), and palisade parenchyma cells (f)
• Light green: spongy parenchyma cells (g) and bundle sheath (k)
• Blue: xylem (i)
• Orange: phloem (j)
• Tan: sclerenchyma fibers (m) and endodermis (r)
• Yellow: resin duct (p)

The leaf blade's outer surface is epidermal tissue and the ground tissue within is photosynthetic mesophyll, interspersed with "veins" (vascular bundles) of conducting (vascular) tissue. Vascular connections between blade and stem pass through the petiole.

Dicot Leaf

Epidermis: There may be one or more layers of epidermal cells (a) with cutinized walls which form a tough outer surface. Stomata of guard cells (b) may be present on either leaf surface, but more are always found on the lower surface.

Ground Tissue: The parenchyma cells of the ground tissue (c) are divided into palisade (d) and spongy (e) mesophyll. Usually the columnar cells (f) of the palisade layer lie under the upper leaf epidermis and contain most of the chloroplasts of the ground tissue. The spongy parenchyma (e) has irregular-shaped cells (g) with air spaces (h) between.

Vascular Tissue: Four small vein transections are shown in the diagram which are part of the vast, interconnecting network between major veins. A leaf vein (vascular bundle) usually has xylem (i) on the upper side and phloem (j) on the lower side. The reverse may be true in major veins. Small veins are usually enclosed by a layer of parenchyma cells called a bundle sheath (k), which functions in moving materials between the vein and the mesophyll tissue.

Monocot Leaf (Grass)

Epidermis: Along with the epidermal cells (a), guard cells (b), hairs, silica, and cork cells, grass leaves have bulliform cells (l). These are bubble-shaped, water-filled epidermal cells that coontrol leaf rolling and unrolling. Under drought conditions, bulliform cells collapse and the leaf rolls up.

Ground Tissue: In the temperate region, grass mesophyll (c) consists of only spongy parenchyma.

Vascular Tissue: Grass leaves have bundle sheaths (k) surrounding each vein. Sclerenchyma fiber strands (m) may be associated with the veins.

Conifer Leaf

Epidermis: The usually small, triangular or flat "needle" leaves of conifers are adapted to dry conditions. Epidermal cells (a) have thick walls of cuticle, and sunken guard cells (b) overlapped on the surface by subsidiary cells (n). Stomata may be on one or all sides of the leaf surface. Under the epidermis may be a layer of thick-walled sclerenchyma cells called hypodermis (o).

Ground Tissue: Mesophyll tissue (c) is not usually differentiated into palisade and spongy layers. Resin ducts (p) are present.

Vascular Tissue: There are one or two veins in the center surrounded by transfusion tissue (q) and endodermis (r) containing tannins and resins.

Color Biogram #18, answer these questions and add both of them to your portfolio.

1. What tissues make up the "veins" of a leaf?
2. Where are most of the stomata found on a dicot leaf?
3. Where are most of the chloroplasts found in a dicot leaf?
4. What is the function of bulliform cells in a monocot leaf?
5. What purpose does the shape of conifer leaves serve?